Page 253 - The Veterinary Care of the Horse

Causes

• Equine metabolic syndrome (see page 648). Obese animals are particularly at risk,
VetBooks.ir along with some genetically predisposed breeds. The combination of excessive rations

and limited exercise causes affected ponies and horses to deposit large amounts of fat on
the crest of the neck, over the shoulders and loins, at the head of the tail and inside the
abdomen. The fat in the abdomen secretes hormones called adipokines and an enzyme

that activates cortisol, both of which alter the animal’s metabolism. They become
resistant to insulin, which is secreted by the pancreas and normally lowers the blood

sugar by encouraging its uptake into tissues such as the muscles and liver. This causes the
production of more insulin so affected animals have high levels of insulin and glucose in

the blood even after fasting. High circulating levels of blood glucose, insulin and cortisol
adversely affect the blood supply to the feet in ways that are not yet fully understood, and

can trigger the onset of laminitis.
• Pituitary pars intermedia dysfunction (PPID), also known as Cushing’s disease (see

page 644). Animals affected by this disease have high circulating levels of cortisol;
insulin and glucose may also be raised, making them prone to developing laminitis.

• Administration of corticosteroids can cause laminitis in some animals and may occur
after systemic treatment, e.g. for skin disease, recurrent airway obstruction or local

injection into a joint. The mechanisms are not fully understood. They may constrict the
blood vessels supplying the feet, affect the action of insulin (causing insulin resistance

and high circulating insulin levels) or have a direct effect on the laminae.
• Any severe infection leading to toxaemia can result in laminitis, e.g. endometritis

arising from a retained placenta following foaling, a serious colic or diarrhoea, other
causes of gut inflammation, pneumonia or pleurisy. The toxins activate a cascade of

events, which cause inflammation in the laminae and damage to the blood vessels that
supply them.

• Excessive weight-bearing on one limb, e.g. arising from a severe strain or fracture of

the opposite limb can lead to laminitis in the uninjured, supporting leg. The pedal bone in
these animals often sinks rather than rotates as a result of trauma and mechanical factors.
The blood supply to the foot may be reduced by the continuous weight-bearing leading to

inadequate perfusion of the laminae.
• Concussion from too much work on hard going can cause laminitis as a result of

mechanical stresses on the laminae.
• Grain overload. Thankfully this is a relatively rare event. If a horse or pony gets into a

feed store and eats a large amount of concentrates, the carbohydrate that is normally